Ultra high frequency antenna



Nov. 28, 1950 R. G. scHRlEFER ULTRA HIGH FREQUENCY ANTENNA Filed April28, 1947 R OR TE NF EE V m NH IC S G. T R E B O R ATTORNEY Patented Nov.28, 1950 UNITED STATES PATENTv OFFICE ULTRA HIGH FREQUENCY ANTENNAApplication April 28, 1947, Serial No. 744,435

(Cl. Z50-33) 6 Claims.

This invention relates to antennae and particularly to an ultra-highfrequency vertical antenna adapted for use in mobile communications.

For railroad radio communications an antenna is required which has a lowclearance. The space limitations imposed on a railroad antenna requirethat the antenna be mounted in close proximity to the metallic vehiclebody which may cause distortion of the radiation pattern, and may reducethe eld strength due to the absorption of a portion of the radiantenergy by adjacent metallic surfaces.

Furthermore it has usually been required in Y antenna installations toprovide an impedance matching device for the purpose of obtaining anecient energy transfer between the radiating element and the antennafeeder line. Alternatively, the length of the antenna may be varieduntil the desired resistive component is obtained, whereupon thereactive component may be tuned out. These conventional arrangementsnecessarily complicate the installation and adjustment of the antennaand often cause ineicient operation due to the instability, ormechanical slippage of the tuning or impedance matching components.Further deficiencies of the prior art antennae reside in the lack ofadequate weatherproofing for the protection of the antenna and itsfeeder line and the absence of suiicient mechanical ruggedness requiredto withstand the severe vibration and shocks to which a mobile antennais subjected.

it is, therefore, an object of the present invention to provide anultra-high frequency antenna especially adapted for use in mobile radiocommunication services in View of its low vertical dimensions.

Another object of the invention is to provide a self-supporting antennaof great mechanical strength having structural simplicity and Whereinthe elements are dimensioned so as to provide a resonant structurehaving a predetermined input impedance for matching the antennaimpedance to that of the antenna feeder line.

A further object of the invention is to provide an antenna structuredevoid of insulation and substantially impervious to accumulations ofsnow, ice or sleet, and which furthermore provides a weatherproofhousing for the connecting elements contained within the antennastructure.

In accordance with the present invention there is provided an antennacomprising a coaxial transmission line having an inner conductor and anouter conductor. A cylindrical member is provided which has a diameterdifferent from that of the outer conductor, that is, it may be largerthan the outer conductor. This cylindrical member extends beyond thetransmission line. Means are provided for connecting the inner conductorto an extremity of the member and further means are provided formechanically supporting the cylindrical member.

For a better understanding of the invention, together with other andfurther objects thereof, reference is made to the following'description,taken in connection with the accompanying drawing, and its scope will bepointed out in the appended claims.

In the drawing, the single figure is a cross sectional view of avertical antenna embodying the present invention.

Referring now to the drawing, there is illustrated a coaxial verticalantenna comprising outer cylinder I and inner cylinder I I. The upperrim of outer cylinder I is aiiixed to a metallic plug i2 which is, forexample, screwed upon the upper rim of the inner cylinder Il. The outercylinder iii extends downwardly, in skirt fashion, concentrically With asubstantial portion of the upper length of inner cylinder II. The innercylinder i E serves as a support for the antenna structure and ismounted on base plate I5 and may be fixed thereto by set screw I 8. Thebase plate I5 is then fastened to a ground plane or metallic surface I6by the mounting screws II. Metallic surface I5 is preferably grounded,as shown, thereby to maintain the cylinders II and IQ at direct currentground potential.

By this arrangement, a rigid and self-supporting metallic antennastructure is provided, wherein no insulation is required, and whereinthe entire antenna structure may be maintained at a direct currentground potential.

There is provided a coaxial transmission line comprising the lowersection of inner cylinder I I and the center conductor 20 which serve asan antenna feeder line for supplying alternating excitation current tothe antenna or for feeding alternating current from the antenna to areceiver. Aperture 2i is arranged in the sidewall of inner cylinder II,and the free end of the center conductor 2B extends through thisaperture and is electrically connected to the lowermost portion of outercylinder I0, that is, to its lower eX- tremity.

The electrical length of the antenna extending above metallic surface I6corresponds to one quarter of the operating wave length as indicated inthe drawing. L1 indicates the length of outer cylinder I0, while La isthe length of the exposed portion of inner cylinder II, that is, theportion of inner cylinder II which is not surrounded by outer cylinder Iand which extends beyond metallic surface I6. D1 and D2 indicate thediameter of the outer cylinder I0 and of the inner cylinder; I I;respectively.

The inductive and capacitive reactive components and the radiationresistance of the coaxial antenna of the invention are mainly dependentupon the ratios of the lengths LinL'z' and ofthe,-

diameters D1, D2.

The antenna of the invention-isxsubstantially equivalent to a verticalquarter wave dipole and accordingly will have an omnidirectional'horizontal radiation pattern and will radiate a vertically polarizedwave. A high frequency excitation current applied to the antenna throughthe coaxial feeder line II, 2i] will excite the outer surfaces of innercylinder II and o'outer cylinder--Ill -which together form a resonantstr-ucture thereby to establish an omnidirectional horizontal radiation-eld Yabout the antenna.

When the antenna is excited; high frequency currents will be presentonfthey outer surface of outer'vcylinder I0 `as Well asl upon theouterfsuriace of theexposedportion-of rinner cylinder II. These currentsmust be in phasefand are indicate'd-for a giveninstant by arrows 22 and23. At thevsarnetime the'high frequency currents flow through coaxialtransmission line vII, 2S. Thus, acurrent indicated loyal-row 2li flowsalong thefouter-surfa'ce of inner conductor 2Q in the same direction asthe current llovvin;r on the z..

outer surfaces of cylinders Iii and II. Since the exposed portion ciinner cylinder II forms part of the coaxial transmission line, a currentindicated by arrow will ow on the inner surface of cylinder I I intheopposite direction from that flowing-'on its outer surface and indicatedby arrow 23. The currents distributed over the comn bined outerconducting -surfaces'of cylinders II andfl -as indicated by arrows 22vand 23 for a certain instant-causethe structure to resonate and tofunction as an antenna. It is to be understood that the currentsrepresented by arrows 22 `to-25-willfilow -in the reverse directionduring the opposite portion of a cycle of operation.

The portion-of cylinder iI varranged within outercylinder IB is notimpertantfor the operation or the antenna but'mainly serves as asupportfor the outerl cylinder IG.- However, the electricalconnectionestablished from the metallic surface I8 through innercylinder II,metallic plug I2 to outer cylinder` I@ keeps the entire antennastructure at a direct current ground potential.`

Experiments yhave revealed that when the entire physical. length oftheantenna extending.'

beyond metallic surfaced@ is kept constant, while the length L1 of outercylinder Ill is reduced, whichsi-multaneously,will increase the lengthL2 of the exposed portion of inner cylinder II, the antenna resonates ata slightly higher frequency \vhileth'e antenna input impedanceissimultaneously increased. Inversely,v if the length L1 is increasedwhile the `length L2 -is decreased and the total length of the antennamaintained con-- stant, the antenna will* assume a slightly lowerresonant frequency and the antenna input impedance will be decreased.

When-the dimensions L1, L2 and Dz'are maintained constantlwhilethediameter D1 of outer cylihderelll is reduced,` the antenna resonatesat a slightly higher frequency While the antenna input impedance isincreased. A decrease of diameter D2, that is, the diameter of innercylin der II causes a reduction of the resonant frequency of the antennaas well as of the antenna input impedance provided that L1, L2 and D1are kept constant.

B3n-suitably choosing ,the ratios .of the lengths Li, Lz' and of thediameters D1, D2', the antenna input impedance may be adjusted to matchthe impedance of the coaxial transmission line which feedsj-theantenna.y Accordingly, an impedance matching network is not requiredbetween the feeder line and the antenna. The dimensions of the antennaofthe invention suitable for operation on a frequency of approximatelymegacycles .and having an antenna input impedance of 50 ohms are asfollows:

Inches Inches L2 3%; D2v 1 Forf-fprovidingfan antenna which willresonate at a frequency different from 160 megacycles and which will`have the `same antennainput 'impedance 4of 50. ohms, theabove dimensionsmay be Vmultiplied byrthe ratio ofith'e desired fref quency to the givenfrequency of 160 megacycles.

While there has been.- described what is at present considered thepreferred embodimentl of theA invention, it will be obvious tothoseskilled in the .art that 'various changes andmodications may be madetherein without` departing from the invention,` and it is,therefore,aimed in the appended vclaims to cover allllsuch changes andmodificationsk as fail withinl the true spirit and scope of-theinvention.

What is claimed is z l. An antennacomprising a coaxial transmission'line having an inner conductor and an outer conductor, acylindricalmember surrounding a portion ofsaid'outer conductor, a connectingelement for electrically and mechanically connecting one extremity ofsaid member to said outer conductor, means for connecting the other endof'said member to said'inner conductonand a metallic surfaceelectricallyvconnected to said outer conductor, the length of saidantenna extendingI beyond said surface being electrically equivalent toone-quarter of the operating waveA length or"y said antenna.-

2. An antenna comprising al coaxial transmission line having an innerconductor and an outer f conductor, a Acylindrical member coaxial withand surrounding the Youter portion of said outerv conductor, aconnecting element for electrically and mechanically interconnecting theupper extremities of said member and of said outer conductor,meansffor-connecting the free end of said inner conductor to thelowermost portion of said member, and a metallic surface electricallyconnected' to saidouter conductor, the length of said antenna extendingbeyond said surface being electrically equivalent to one quarter of theoperating'wave length of said antenna.

3. A vertical antenna comprising a coaxial transmission line having aninner conductor and an outer conductor, a-cylindrical member coaxialwith and surrounding the upper portion of said outer conductor, meansiorconnecting the free end-of said inner conductor to the lowermost portionof said member, anda connecting element for electrically andmechanically interconnecting-,the upperV extremities .of said 'memberand of said outer conductor, the relative length of said member and ofthe radiating portion of said outer conductor not surrounded by saidmember being proportioned in such a manner as to provide a predeterminedinput impedance of said antenna and a predetermined electrical length.

4. A vertical antenna comprising a coaxial transmission line having aninner conductor and an outer conductor, a cylindrical member coaxialwith and surrounding the upper portion of said outer conductor, aconnecting element for electrically and mechanically connecting theupper extremities of said member and of said outer conductor, means forconnecting the lowermost portion of said member to the free end of saidinner conductor, and a surface electrically connected to said outerconductor, the length of said antenna beyond said surface beingelectrically equivalent to one quarter of the operating Wave length ofsaid antenna, the relative length of said member and of the radiatingportion of said outer conductor arranged between said surface and saidmember being pre portioned in such a manner as to provide apredetermined input impedance of said antenna and an electrical lengthequivalent to one quarter of said wave length.

5. A Vertical quarter-wave antenna comprising a coaxial transmissionline having an inner conductor and an outer conductor, a cylindricalmember coaxial with and surrounding the upper portion of said outerconductor, means for connecting the free end of said inner conductor tothe lowermost portion of said member, and a connecting element forelectrically and mechanically interconnecting the upper extremities ofsaid member and of said outer conductor, the diameter of said member andof said outer conductor being such as to provide a predetermined inputimpedance of said antenna and a predetermined electrical length.

6. A vertical antenna comprising a coaxial transmission line having aninner conductor and an outer conductor, a cylindrical member coaxialwith and surrounding the upper portion of said outer conductor, aconnecting element for electrically and mechanically connecting theupper extremities of said member and of said outer conductor, means forconnecting the lowermost portion of said member to the free end of saidinner conductor, and a surface electrically connected to said outerconductor, the length of said antenna beyond said surface beingelectrically equivalent to one quarter of the operating wave length ofsaid antenna, the diameter of said member and of said outer conductorbeing such as to provide a predetermined input impedance of said antennaand an electrical length equivalent to one quarter of said wave length.

ROBERT G. SCHRIEFER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,113,136 Hansell et al. Apr. 5,1938 2,234,234 Cork et al. Mar. 11, 1941 2,284,434 Lindenblad May 26,1942 2,321,454 Brown June 8, 1943 2,323,641 Bailey July 6, 1943 FOREIGNPATENTS Number Country Date 260,005 Great Britain Mar. 24, 1927

